A fundamental structure of a liquid crystal display device is commonly one in which a polarizing plate is provided on both sides of a liquid crystal cell. Since a polarizing plate passes only light of a polarized wave plane from a predetermined direction, it plays a significant role in visualizing variations of the orientation of a liquid crystal via an electric field in a liquid crystal display device, whereby performance of the liquid crystal display device largely depends on performance of the polarizing plate.
Over recent years, with the increased size of the screen of liquid crystal panels, wide polarizing plate protective films and retardation films have been urgently sought.
Conventionally, in manufacturing an optical film via a solution casting film forming method, a resin solution containing a desired component is cast on a rotationally-driven endless metal belt (belt support), dried to a conveyable state, peeled from the belt support, and then dried, followed by winding as a prepared film.
Herein, as a method for preparing a wider film, there is cited a manner of transverse stretching of a peeled film, however, which have ever produced problems that required optical characteristics were not realized; unviable or application-limited products resulted due to variation in mechanical characteristics.
Therefore, conventionally, a wide film has been manufactured by expanding the casting width via increase of the width of the belt support.
In contrast, to control meandering during film (web) conveyance using a belt support, the angle created by the rotational axes of a pair of front and rear rotating drums rotationally driving an endless metal belt is allowed to vary. For example, a method is employed in which the rotational axis of either one of the rotating drums is changed, to some extent, from a right angle to the moving direction of the belt support.
However, in such meandering control of the belt support, since strong force is loaded to belt end portions, repetitive control tends to deform the belt end portions. Especially, when the width of the belt support exceeds 1.8 m, such deformation becomes pronounced. With large deformation, the contact between the belt end portions and a heating/cooling drum becomes weak, between which heat transfer is decreased, resulting in occurrence of incomplete drying of the end portions and bubble defects at the end portions. It has been noted that the problem of incomplete drying of the belt end portions resulted in incomplete peeling wherein a part of the film remained on the belt during peeling; and when the right and left end portions of the belt support were further largely deformed, the flatness of film products became deteriorated and the apparatus was damaged via contact between the belt support end portions and the casting die, resulting in adverse effects during film manufacturing.
Herein, a patent document relating to a conventional method for manufacturing an optical film via a solution casting film forming method employing a wide belt support is as follows:
Patent Document 1 discloses a solution film forming method, and describes that a belt featuring a width of at least 1.6 m is used as a support to support a polymer solution film cast from a die; the amount of meandering of the belt is controlled to be at most ±0.5% based on the belt width; and also the casting width of the film from the die is controlled to be at least 70% of the belt width, as well as being controlled to be at most [belt width−(meandering amount×4)].
Patent Document 1: Unexamined Japanese Patent Application Publication No. (hereinafter, referred to as JP-A) 2002-127169